Intramedullary nails (bone nails) have become a treatment of choice for the fixation of bone fractures, especially fractures of long bones (e.g., the humerus, tibia and femur). Typically, bone nails are rod-shaped devices configured and constructed to be secured (interlocked) to a bone using one or more locking elements, such as transverse screws at one or both ends of the nail.
In many cases, the implant is constructed from metal, such as titanium, stainless steel or cobalt chromium. Although metallic implants provide numerous advantages, they also have a few drawbacks. Metal construction normally provides adequate bending strength, thus reducing problems associated with implant fracture and fatigue. However, the rigid metal implant creates a relative high degree of stresses in certain regions of the bone, while, on the other hand, does not provide for sufficient load transfer resulting in stress shielding. Both high stress and stress shielding can cause bone deterioration and resorption, leading to areas of bone weakness and loss of bone support for the implant (e.g., intramedullary nails and stem components of joint replacement systems). In addition, metals may result in artifacts in CT and MR imaging. Furthermore, metals such as stainless steel and cobalt chromium may cause biocompatibility problems related to corrosion and sensitization reaction (mainly due to allergy to nickel).
Non-metal implants made of a lighter and more flexible material, yet having sufficient strength for load bearing, have been suggested in the past. In particular, composite material implants, for example formed of polymer reinforced with fibers, are discussed in U.S. Pat. Nos. 4,750,905, 5,181,930, 5,397,358, 5,009,664, 5,064,439, 4,978,360, 7,419,714 the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,009,664 describes a tubular, curved marrow nail, made of carbon fibers, which are preferably knit in a crisscross fashion, saturated in a hardenable plastic, with a conically tapered distal tip.
U.S. Pat. No. 5,181,930 describes an implant comprising an elongated core formed of continuous filament fibers embedded in thermoplastic polymer. The core is encased within a filler, made of a non-reinforced polymer which is molded around the core to proximate the final desired shape of the implant. A sheath, composed of reinforced fibers embedded in a polymer, is spiral wound around the filler, at angles (orientations) which may vary along the implant axis.
Although composite material implants can provide several advantages, they also have a few limitations. In contrast to metal, composite material implants are not visible under imaging devices (such as fluoroscopy), and hence their implantation as well as tracking during follow-up are difficult. U.S. Pat. No. 7,419,714 describes a bone screw or plate formed of a composite of polymer or ceramic material with reinforcing fibers, in which at least part of which are made of an X-ray absorbent material. For bone nails or plates, accurate insertion of the screws into the holes in the nail/plate is crucial to the success of the operation, especially where no aiming device is used. The use of interlocking screws poses a problem in such implants, as the designated holes at the nail ends (or at the plate), through which the screws are to be introduced, are not visible under fluoroscopy. The addition of fibers made of material that absorbs X-rays may be insufficient; as such fibers often do not adequately and accurately mark the hole. Also, in order to improve the visualization of implant hole a large quantity of such fibers might be required. In addition, with regards to intramedullary nails (or other implant construction that may comprise a weakened area), due to the composite material construction, the extremities of the nails at the area of the interlocking screw holes are more prone to damage.
Further, although such composite materials may have several properties that are claimed to be similar to those of bone, the composite material construction may be less efficient under torsion loads.
Additionally, the instrumentation that is used with a metal implant, such as an insertion handle, is usually connected to the implant via a thread at a proximal end of the implant. However, the composite material construction (which is not isotropic as is metal), has less resistance to shear forces, and damage (e.g., breakage) may result at the thread area.
The present invention addresses improvements in the above-noted areas, and in other areas of composite bone implant technology.